In many cases, characteristics of a concrete structure are described by using Hooke's law. This approximation is justified in such cases where a concrete structure is exposed only to stresses that are small in relation to its present compressive strength and where the impact of loading sets in at a relatively late point in time. In case of early load impacts at high pressure levels, the rheological behavior of the material must also be considered in any static calculation.
It is known that, given certain conditions, concrete will present a clearly plastic deformation behavior, and that time-dependent deformations will sometimes amount to a multiple of the instantaneously occurring plastic deformations. For certain applications, for example when measuring characteristics of shotcrete layers in tunnel constructions, this fact is of essential significance for the results of the calculations and thus for the safety and economy of the structure.
On account of the many parameters that have an influence upon each other, the computing assumptions must be checked against measurements on the actual structure. The widest possible coverage of all deformations (e.g. extensions) at selected locations is of particular consequence in this connection.
It is known that extension measuring-value sensors with pipe-shaped deformation members, as support elements for extension-measuring strips, are used to measure extensions in concrete structures. As a rule, such sensors have a higher extension stiffness than the material column displaced by them, particularly when using steel pipes of small diameter. This prevents the lossfree transfer of extension deformation from the concrete to the support element of the extension-measuring strip.
It is also known from Federal German Patent DE-PS No. 3 101 575 that extension-measuring sensors can have hollow profiles as deformation members, where the cross-sectional surface, defined by the outer shape of the hollow profile, and the cross-sectional material surface of the hollow profile are chosen as a relation between the elasticity modulus of the material of the support member and the elasticity modulus of the concrete. This serves the purpose of obtaining an extension stiffness value for the sensor which equals the extension stiffness value of the displaced concrete so that the deformation properties within the range of the sensor are not varied, thereby preventing distortions of the transferred forces by the sensor.
With this construction, however, it must be borne in mind that the strength and deformation characteristics of a concrete will change in dependence upon time and stress. It is therefore possible only at one single point in time to obtain undistorted measuring values. This construction does therefore not permit measurement at an early point in time and does not take account of the time-dependent deformation parameters of the concrete. Also, creep extensions cannot be sensed without losses.
It is also known from German Democratic Republic Patent DD-PS-No. 141 710 to embody the support member as a curving bent sheet which is located in a straight sleeve. In this case, measurements can be taken only, if the bent sheet can curve without obstruction. The hollow space required for this purpose on one side runs the risk of becoming filled with moisture or water, especially when applied outdoors, in particular in connection with long-performance underground structures or under rough construction-site conditions. In the long run, this results in the risk of corrosion of the thin bent sheet, or its connections, and the risk of obstructing the free deformation of the bent sheet by ice or sinter forming in the course of time, which entails uncertainties, mainly in the event of long-term measurements outside the laboratory, without the possibility of repairing the damage.
With regard to the use of deformation sensors having a lower extension stiffness value than that of the material column displaced by it, experts in the field have maintained so far mainly that, on account of an impact upon the transferred forces in the area of measurement, correspondingly distorted values only are measured.